Flame retardant n,n&#39;-bis(tetrabromophthalimide) composition

ABSTRACT

This invention relates to a process for preparing and treating a product predominant in N,N&#39;-bis(tetrabromophthalimide) so as to produce a bisimide product which has enhanced suitability for use in thermoplastic formulations as a result of reduction in volatile impurities and particle size.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing a thermally stableflame retardant product which is predominant in the bisimide, ##STR1##

N,N'-bis(tetrabromphthalimide) predominant products have been found tobe useful flame retardants in polyesters, e.g. polybutyleneterephthalate, and other resin formulations. While various processeshave been described for the preparation of such bisimide predominantproducts, almost all produce a product which contains significantamounts of volatile impurities, which impurities contribute to theproduct's lack of good thermal stability. See, for example, the processof Bonnet et al. U.S. Pat. No. 4,894,187 wherein aN,N'-bis-(tetrabromophthalimide) product is produced which contains upto 80% by weight of a substantially equimolar amount of the impurities,tetrabromophthalic anhydride and N-aminotetrabromophthalimide. Thetetrabromophthalic anhydride impurity is especially troublesome as itvolatilizes at polymer processing temperatures.

THE INVENTION

This invention provides a process for enhancing the suitability of abisimide product for use in thermoplastic formulations the bisimideproduct being predominant in N,N'-bis(tetrabromophthalimide) andcontaining, as an impurity, tetrabromophthalic anhydride. The processcomprises: contacting the bisimide product with an aqueous basicsolution; and removing at least a portion of any reaction productsproduced by the contacting to yield, as a remainder, the bisimideproduct having said enhanced suitability.

The process of this invention enhances the suitability of the so treatedbisimide product by (1) reducing the volatile impurity content of thebisimide product and (2) reducing the average particle size of thebisimide product. The former results in a more thermally stable bisimideproduct and thus one which is more acceptable in the work environment.The latter is important as a smaller particle size helps insure evendistribution of the bisimide product in a thermoplastic formulation.Even distribution is important as maximization of the flame retardantproperty in the thermoplastic formulation is achieved.

The process of this invention may be performed on the bisimide productat any point during its production which is after completion of thebisimide forming reaction. For example the process of this invention canbe practiced on the bisimide product before it is separated from thereaction mass in which it was formed, during its separation from thereaction mass, and after its separation from the reaction mass. Theprocess can be performed on a bisimide product after it has undergonepost separation treatment, e.g. drying etc. It is preferred to treat thebisimide product after it has been separated from the reaction mass andis in the form of a wet cake. For example when the bisimide product isseparated from the reaction mass by centrifugation, the resultant wetcake can be contacted with the aqueous basic solution before dischargeof the wet cake from the centrifuge. In a particularly preferredembodiment, the wet cake is washed with water in the centrifuge prior tocontacting it with the aqueous basic solution.

If the process of this invention is performed on a fully recovered dryproduct, an aqueous slurry of the product can be prepared which is thencontacted with the aqueous basic solution.

In the process of this invention, the aqueous basic solution may be anybasic solution compatible with N,N'-bis(tetrabromophthalimide), such asNaOH, KCH, NH₄ OH, Na₂ CO₃ solutions and the like. Particularlypreferred is a NH₄ OH solution. The basic solution may also be providedby bubbling gaseous ammonia through an aqueous slurry containing thebisimide product.

The bisimide product is ccntacted with an amount of the aqueous basicsolution and for a period of time which are sufficient to obtain theenhancemert sought for the bisimide product. A convenient method fordetermining the sufficiency of the contacting is to monitor the drop inthe acid number, expressed as milligrams (mg) of KOH per gram (g) ofbisimide product neutralized, during the contacting period. A drop inthe acid number of about 40% or greater will signify a substantialreduction in the tetrabromophthalic anhydride content and thus agenerally significant increase in the thermal stability of the bisimideproduct. For bisimide products of the type described in U.S. Pat. No.4,894,187 which contain tetrabromophthalic anhydride and N-aminoimide inequimolar and in significant amounts, a reduction in the acid number offrom about 50% to about 90% will be indicative of suitable contactbetween the aqueous basic solution and the bisimide product. Sinceenhancement of the bisimide product, in accordance with this invention,is obtained for any increase in thermal stability and/or any averageparticle size reduction, the above percentages for the acid number droprepresent preferred drops and are not to be taken as absoluterequirements to be achieved to obtain any of the benefits of thisinvention. For example, an acid number drop of 10% or 20% may not be assignificant as the above larger acid number drops, but that should notlead to a conclusicn that no increase in thermal stability and/orreduction in average particle size is achieved. On the contrary, in somecases, small drops in the acid number may be indicative of just theparticular enhancement of suitability sought for a particular bisimideproduct treated.

After contacting the bisimide product with the aqueous basic solution,the so contacted bisimide product is recovered after separating at leasta portior of any of the reaction products produced by the contactingfrom the so contacted bisimide product. The reaction prcducts arepredominantly the reaction products of tetrabromophthalic anhydride andthe aqueous basic solution. Removal of the reaction products from the socontacted bisimide product can be accomplished by conventional means,such as washing of the contacted bisimide product with water followed bya liquid-solid separation technique, e.g. decantation, settling,filtration, centrifugation and the like.

The remaining bisimide product is then dried by conventional means. Thedry bisimide product will be of a particle size which will not requireperformance of subsequent size reduction techniques, e.g. grinding,milling, etc. or will only require the use of such techniques to anextent which is less that that which would normally be required.

The dry product may contain other impurities, such as N-aminoimide,which are less likely to adversely affect the thermal stability of thebisimide product as such impurities are present in very small amounts orare less volatile than tetrabromophthalimide. The N-amincimide impuritymay be represented by the following formula: ##STR2##

In another embodiment of the invention, a product predominant inN,N'-bis(tetrabromophthalimide) and having non-equimolar amounts oftetrabromcphthalic anhydride and N-aminoimide as impurities is producedby: (a) forming a reaction mass from at least (i) tetrabromophthalicanhydride, (ii) hydrazine or a hydrazine provider compound, and (iii) areaction medium selected from a concentrated sulfuric acid medium, anoleum medium, and an aqueous medium; (b) maintaining the so-formedreaction mass at a temperature within the range of from about 40° C. toabout 300° C. for a time sufficient to obtain a product predominant inN,N'-bis(tetrabromophthalimide) and having substantially equimolaramounts of tetrabromophthalic anhydride and N-aminoimide; (c) separatingt.he product from the reaction mass; and (d) contacting the product withan aqueous basic solution.

The tetrabromophthalic anhydride used in the process of this inventioncan be obtained in a very pure form, e.g. 99⁺ % pure from EthylCorporation as Saytex RB-49 flame retardant.

The hydrazine component used in forming the reaction mass can besupplied by hydrazine itself or by hydrazine providing compound, i.e., ahydrazine salt, hydrate, etc. which, in the reaction mass environment,will provide hydrazine. Suitable hydrazine providing compounds arehydrazine sulfate, hydrazine hydrate, hydrazine monohydrate, hydrazinedihydrochloride, hydrazine monohydrochloride, hydrazine tartrate,hydrazine acetate, and hydrazine bisulfate. Mixtures of varioushydrazine salts, i.e. hydrates, sulfates, acetates, tartrates,hydrochlorides, etc., may also be used. Preferred is hydrazine sulfate.

In a particularly preferred embodiment of the invention, concentratedsulfuric acid is used as the reaction medium. When used as the reactionmedium, it is generally a 90 to 100% sulfuric acid medium. Thecommercially available concentrated acids are preferred, e.g. thosefalling within the range of 93 to 100% sulfuric acid. Some specificexamples are, 93%, 96%, 98-99% and 100% sulfuric acid.

In forming the reaction mass, it is convenient to first charge areaction vessel with a solution of tetrabromophthalic anhydride and afirst portion of ccncentrated sulfuric acid and then adding, to thisso-charged solution, a solution comprised of the hydrazine component anda second portion of concentrated sulfuric acid. The first and secondportions of concentrated sulfuric acid substantially equal the totalacid used in forming the reaction mass. Generally, the first portionwill comprise 40% to 60% of the total amount of concentrated sulfuricacid used. In another preferred method of formation, the same solutionsare used but the order of addition is reversed. While these methods arepreferred, other techniques can be used to form the reaction mass. Forexample, the hydrazine component can be added neat to a solutioncomprised of the tetrabromophthalic anhydride and all of theconcentrated sulfuric acid to be used in forming the reaction mass.

The total amount of sulfuric acid used in forming the reaction mass iswithin the range cf from about 5 to about 50 moles of acid per mole oftetrabromophthalic anhydride. A preferred amount is within the range offrom about 8 to about 12 moles per mole of tetrabromophthalic anhydride.

When an aqueous medium is used in forming the reaction mass it will beused in the same manner and in the same amounts as are taught for theN,N'-bis(tetrabromophthalimide) process of

U.S. Pat. No. 4,894,187, which is incorporated herein, for theirteachings, as if fully set forth. As taught in U.S. Pat. No. 4,894,187,the aqueous medium may be comprised of water and minor amounts of acid.

When concentrated sulfuric acid is used as the reaction medium, thereaction mass is formed at a temperature within the range of from about80° C. to about 270° C. and preferably within the range of from about180° C. to about 230° C. It is preferred that the molar ratio oftetrabromophthalic anhydride to the hydrazine or hydrazine providingcompound used in forming the reaction mass be from about 1:0.6 to about1:0.9. These ratios provide an excess over the stoichiometric ratio of1:0.5, which excess provides sufficient amount of hydrazine to reactwith substantially all of the tetrabromophthalic anhydride. To obtain aspure a product as is possible at the highest yield, the practitionershould adjust this ratio so that the amount of unreacted hydrazine andtetrabromophthalimide anhydride is minimized. It is possible to use astoichiometric deficiency or excess of tetrabromophthalic anhydride andstill produce a N,N'-bis(tetrabromophthalimide) product, however, theobtained product will contain, respectively, monoimides of hydrazineand/or tetrabromophthalic anhydride.

After the reaction mass has been substantially formed, the reaction massis maintained at a temperature within the range of from about 110° C. toabout 300° C., and preferably maintained at a temperature within therange of from about 180° C. to about 230° C. until the reaction issubstantially complete. The reaction mass may be maintained at theselected temperature for a period up to about 20 hours and preferablyfrom about 6 to about 14 hours in order to substantially oomplete thereaotion. This period is shorter when using the higher temperatures andlonger when using the lower temperatures.

The process pressure is preferably atmospheric, however, sub-atmosphericand superatmospheric pressures can be used provided that care is takento prevent loss of any of the major constituents of the reaction mass.

Subsequent to this period, the bisimide product is separated from thereaction medium and is in the form of a wet cake. The wet cake is thenwashed with a washing medium to remove residual sulfuric acid from thewet cake. After washing, the wet cake is treated by contacting the wetcake with an amount of an aqueous NH₄ OH solution sufficient to obtain areduction in the acid number of the so treated bisimide product. It ishighly desirable to obtain a reduction in the acid number of the sotreated bisimide product of about 40% or greater in order tosignificantly increase the thermal stability of the bisimide product.

As in the case for the first described embodiment of the invention, thesecond embodiment results in the reduction of the amount of volatileimpurities in the product and, surprisingly, a reduction in the averageparticle size. Since the particles have a smaller size, less milling orgrinding will be required to meet particle size specifications.Typically the average particle size of the bisimide product produced bythe process of this invention is less than about 6 microns and at leastabout 99% of the particles are typically less than about 16 microns. Theparticle size may vary however, depending on the average size particlesproduced prior to the contacting step.

The enhanced bisimide product is further characterized in that it has anon-equimolar mixture of the impurities tetrabromophthalic anhydride andN-aminoimide. On a molar ratio basis, the bisimide product may have aratio of from about 1.5:1 to about 100:1 moles of N-aminoimide per moleof tetrabromophthalic anhydride. Typically, the molar ratio ofN-aminoimide to tetrabromophthalic anhydride ranges from about 2:1 toabout 10:1 as a result of the process of this invention.

After treating the wet cake with the aqueous NH₄ OH solution, the wetcake is filtered to recover the solids. Removal of residual aqueous NH₄OH solution from the filtered solids can be accomplished by againwashing the solids with a washing medium. The washing medium which isused before and after the contacting step, is preferably water, butother mediums such as methanol, acetone, ethanol may be used.

Subsequent to washing the solids to remove residual aqueous NH₄ OHsolution, the solids are dried to remove any remaining washing medium.The filtration, washing, contacting, rinsing, and drying steps are alldone conventionally.

The equipment in which the process of this invention is carried outshould be of materials which can withstand the corrosive nature of thecompounds with which it may come into contact. For example, glass-linedequipment is especially suitable for the process of this invention.

The bisimide products obtained in accordance with the processes of thisinvention are particularly well suited as a flame retardant in plasticsof all kinds. These bisimide products may be used as a flame retardantin formulation with virtually any flammable material. The material maybe macromolecular, for example, a cellulosic material or a polymer.Illustrative polymers are: olefin polymers, cross-linked, and otherwise,for example, homopolymers of ethylene, propylene, and butylene;copolymers of two or more of such alkylene monomers, and copolymers ofone or more of such alkylene monomers, and any other copolymerizablemonomoners, for example, ethylene/propylene copolymers, ethylene/ethylacrylate copolymers, and ethylene/vinyl acetate copolymers; polymers ofolefinically unsaturated monomers, for example, polystyrene, e.g. highimpact polystyrene, and styrene copolymers; polyurethanes; polyamides;polyimides; polycarbonates; polyethers; acrylic resins; polyesters,especially poly(ethyleneterephthalate) and poly(butyleneterephthalate);epoxy resins; alkyl resins; phenolics; elastomers, for example,butadiene/styrene copolymers and butadiene/acrylonitrile copolymers;terpolymers of acrylonitrile, butadiene, and styrene; natural rubber;butyl rubber; and polysiloxanes. The polymer may also be a blend ofvarious polymers. Further, the polymer may be, where appropriate,cross-linked by chemical means or by irradiation. Its incorporation intothese substances may be carried out by any known method at doses rangingfrom about 5 to about 40% relative to the weight of the inflammablesubstance.

The following Examples illustrate the process of this invention and arenot to be taken as limiting the scope of the invention. EXAMPLE I

Preparation of N,N'-Bis(Tetrabromophthalimide)

This Example is not of this invention but was run to provide a base linefor comparison. Into a one-liter resin kettle was charged 481.0 g ofSaytex RB-49 flame retardant (tetrabromophthalic anhydride) and 506 g of96.7% sulfuric acid. The resultant solution was stirred with an overheadstirrer and heated to 100° C.

Into a 500 mL Erlenmeyer flask was charged 102.8 g hydrazine sulfate and706 g of 96.7% sulfuric acid. The flask contents were heated and stirredurtil all of the hydrazine sulfate was dissolved.

The hydrazine sulfate solution was then added dropwise to the resinkettle with a polyethylene pipet over a period of sixty-five minutes toform a reaction mass.

The resulting reaction mass was stirred and heated to 80° C. for 14hours. The reaction mass was allowed to cool and then subjected tovacuum filtration using a 3-liter funnel (90 C glass frit) to remove thesulfuric acid. The filter cake was then scraped into a 4-liter beakercontaining 3 liters of ice water. The mixture was stirred, allowed tosettle, and decanted. The filter cake was returned to the 3-literfunnel, washed and filtered until the supernatant water had a neutralpH. The filter cake was then placed in a large crystallizing dish anddried at 100° C. under vacuum for 24 hours to give 449.5 g of a whitesolid (94% yield) that had a melting point >500° C. ThermogravimetricAnalysis (TGA) was used to determine the purity of the solid as 95%N,N'-bis(tetrabromophthalimide).

EXAMPLE II

A batch of N,N'-bis(tetrabromophalimide) was produced by the generalprocedure of Example I except that the product was treated beforedrying. The wet filter cake containing sulfuric acid was split into twofractions. One fraction was washed with water. The dry cake from thefraction washed with water had an acid number of 1.28 mg KOH/g andccntained 92% N,N-bis(tetrabromophthalimide). The second fraction waswashed with water, then treated with a 5% NaOH solution followed by awater wash. The dry cake from the second fraction had an acid number of0.77 mg KOH/g, and contained 94% N,N'-bis(tetrabromophthalimide). Acomparison of the properties of the two fractions is contained in Table1.

                  TABLE 1                                                         ______________________________________                                                                 99%                                                         Acid No. Mean     less TGA %                                                  (mg      Particle than Wt Loss  TGA wt %                               Fraction                                                                             KOH/g)   Size (μ)                                                                            (μ)                                                                             up to 300° C.                                                                   Bisimide                               ______________________________________                                        Water  1.28     7.73     --   3.8      92                                     Washed                                                                        Treated                                                                              0.77     5.44     --   1.6      94                                     ______________________________________                                    

EXAMPLE III

A batch of N,N'-bis(tetrabromophthalimide) was produced by the generalprocedure of Example I except that the product was treated beforedrying. As in Example II, the wet filter cake containing sulfuric acidwas split into two fractions. One fraction was washed with water. Thedry cake from the fraction washed with water had an acid number of 2.15mg KOH per gram and contained 94% N,N'-bis(tetrabromophthalimide). Thesecond fraction was washed with water, then treated with an ammoniumhydroxide solution followed by a water rinse. The acid number of thesecond fraction was 0.26 mg KOH per gram and contained 96%N,N'-bis(tetrabromophthalimide). A comparison of the properties of thetwo fractions is contained in Table 2.

                  TABLE 2                                                         ______________________________________                                                                 99%                                                         Acid No. Mean     less TGA %                                                  (mg      Particle than Wt Loss  TGA wt %                               Fraction                                                                             KOH/g)   Size (μ)                                                                            (μ)                                                                             up to 300° C.                                                                   Bisimide                               ______________________________________                                        Water  2.15     5.64     25.9 3.4      94                                     Washed                                                                        Treated                                                                              0.26     4.94     15.6 0.8      96                                     ______________________________________                                    

EXAMPLE IV

Following the general procedure of Example I, a batch ofN,N'-bis(tetrabromophthalimide) was prepared. The dried productcontained 86% N,N'-bis(tetrabromopthalimide) and had an acid number of1.65 mg KOH per gram. One fraction of the dry product was slurried inwater and the slurry was fed to a centrifuge. The solids were treatedwhile in the centrifuge with an ammonium hydroxide solution and thenwashed with water. After drying this fraction, the product contained 95%N,N'-bis(tetrabromophthalimide) and had a acid number of 0.29 mg KOH pergram. A comparison of the treated and untreated product is contained inTable 3.

                  TABLE 3                                                         ______________________________________                                                                  99%                                                         Acid No. Mean     less TGA %    TGA                                           (mg      Particle than Wt Loss  wt %                                  Fraction                                                                              KOH/g)   Size (μ)                                                                            (μ)                                                                             up to 300° C.                                                                   Bisimide                              ______________________________________                                        Untreated                                                                             1.65     23.0     85.1 10.6     86                                    Treated 0.290    12.35    43.25                                                                               1.5     95                                    ______________________________________                                    

EXAMPLE V

Following the general procedure of Example I, a batch ofN,N'-bis(tetrabromophthalimide) was prepared. Prior to separation of thebisimide product from the reaction mass, a portion of the product wastreated by adding a solution of NH₄ OH to the reaction mass toneutralize the sulfuric acid. After obtaining a neutral mixture, anadditional amount of NH₄ OH was added to the reaction mass so as toobtain a slightly basic solution. The other portion was washed withwater but not treated. Both portions were then filtered separately,rinsed with water and then dried. Table 4 illustrates a comparison ofthe treated and untreated products.

                  TABLE 4                                                         ______________________________________                                                                 99%                                                         Acid No. Mean     less TGA %                                                  (mg      Particle than Wt Loss  TGA wt %                               Fraction                                                                             KOH/g)   Size (μ)                                                                            (μ)                                                                             up to 300° C.                                                                   Bisimide                               ______________________________________                                        Water  1.11     6.36     26.14                                                                              2.4      94                                     Washed                                                                        Treated                                                                              0.14     5.56     15.17                                                                              1.6      95                                     ______________________________________                                    

EXAMPLE VI

Following the general proc:edure of Example I, a batch ofN,N'-bis(tetrabromophthalimide) was prepared. The batch was split intotwo fractions. The first fraction was water washed and dried while thesecond fraction was treated with an NH₄ OH solution during thecentrifugation. Table 5 illustrates the characteristics of each fractionof the batch.

                  TABLE 5                                                         ______________________________________                                                                 99%                                                         Acid No. Mean     less TGA %                                                  (mg      Particle than Wt Loss  TGA wt %                               Fraction                                                                             KOH/g)   Size (μ)                                                                            (μ)                                                                             up to 300° C.                                                                   Bisimide                               ______________________________________                                        Water  1.773    3.16     10.47                                                                              5.1      91.8                                   Washed                                                                        Treated                                                                              0.353    2.59      6.61                                                                              0.5      96.8                                   ______________________________________                                    

The bisimide products of this invention have an excellent white color,which color is advantageous when the products are used in producingarticles which are of a light color or which are white.

We claim:
 1. A flame retardant product predominant inN,N'-bis(tetrabromophthalimide) having (i) non-equimolar amounts oftetrabromophthalic anhydride and N-aminoimide as impurities, and (ii) areduction in acid number from about 50 to about 90% produced by aprocess which comprises:(a) forming a reaction mass from at least (i)tetrabromophthalic anhydride, (ii) hydrazine or a hydrazine providingcompound, and (iii) concentrated sulfuric acid; (b) maintaining theso-formed reaction mass at a temperature within the range of from about110° C. to about 300° C. for a time sufficient to obtain theN,N'-bis(tetrabromophthalimide) predominant product; (c) separating theproduct from the reaction mass by centrifugation; (d) contacting theproduct with an NH₄ OH solution; and (e) subsequent to step (d),separating the so contacted product from the Nh₄ OH solution to yieldsaid product containing an impurity comprising tetrabromophthalicanhydride and an N-aminoimide wherein the molar ratio of N-aminoimide totetrabromophthalic anhydride is substantially greater than 1:1.
 2. Aflameproofing composition comprising bis(halogenated dicarboxylic acidimide) and from about 1 to about 80% by weight of a substantiallynon-equimolar mixture of N-aminoimide and an anhydride of thedicarboxylic acid of the imide.
 3. A flameproofing compositioncomprising N,N'-bis(tetrabromophthalimide) and a substantiallynon-equimolar mixture of N-aminotetrabromophthalimide andtetrabromophthalic anhydride.
 4. A flameproofing composition comprisingN,N'-bis(tetrabromophthalimide) and a substantially non-equimolarmixture of N-aminotetrabromophthalimide and tetrabromophthalic anhydrideand having an average particle size of less than about 6 microns andabout 99% of the particles less than about 16 microns.
 5. A flameproofplastic comprising a flammable plastic and a sufficient amount offlameproofing composition of claim 3 to impart flame retardance to theplastic.
 6. A flameproof plastic comprising a flammable plastic and asufficient amount of the flameproofing composition of claim 4 to impartflame retardance to the plastic.